Due to a recent increase in environmental consciousness, a raw material derived from a biomass has been desired. However, for example, as is especially noticeable in the production of bioethanol, as the raw material derived from a biomass, a raw material such as starch or sugar that competes with foods is used in many cases and accordingly, there has been pointed out such problems that the use leads to an increase in food prices and a decrease in food production or the like. Therefore, attention has been currently attracted to a technique for producing biofuels, biochemicals and others from a cellulosic biomass that does not compete with foods.
Examples of the cellulosic biomass that does not compete with foods include trunks and empty fruit bunches of palm, fibers and seeds of palm fruits, bagasse (strained lees of sugar cane (including high biomass content sugar cane)), rice straws, wheat straws, cobs, stems and leaves of corn, corn residues (corn stovers, corn cobs, corn hulls), sorghum (including sweet sorghum) residues, Jatropha seed coats and hulls, cashew hulls, wood chips, switchgrass, Erianthus, energy crops, etc.
One example of using a cellulosic biomass not competing with foods as a raw material is sugar production by enzymatic saccharification treatment. However, all the above-mentioned cellulosic biomasses contain lignin in addition to cellulose and hemicellulose that can be converted into sugar.
Lignin has a complicated three-dimensional structure, and therefore impedes approach to cellulose by a cellulolytic enzyme in enzymatic saccharification treatment. Consequently, presence of a large amount of lignin in a cellulosic biomass provides a problem that the efficiency of enzymatic reaction could be hardly better.
Given the situation, as one method for sufficiently carrying out enzymatic saccharification treatment, a method of carrying out enzymatic hydrolysis after pretreatment of a cellulosic biomass with aqueous ammonia has been proposed (see PTL 1). In addition, a method of carrying out enzymatic hydrolysis after pretreatment of a cellulosic biomass by vapor explosion has also been proposed (see PTL 2).
However, both the above-mentioned methods are still problematic in point of the yield of glucose to be obtained, and further improvement is desired.